Process for making a thermionic cathode

ABSTRACT

A PROCESS FOR MAKING A THERMIONIC CATHODE FOR USE IN THE EMISSION SYSTEM OF A CATHODE RAY TUBE, THE CATHODE BEING COMPOSED OF A SINTERED PRESSED BODY WHICH IS TO BE THE ELECTRON EMITTING ELEMENT AND WHICH IS FORMED BY PRESSING POWDERED MATERIAL OF WHICH THE BODY IS TO BE MADE INTO A CAVITY OF AN APPROPRIATELY SHAPED CATHODE CUP IN SUCH A MANNER THAT THE EMISSION BODY IS FORMED TO COMMUNICATE WITH THE CATHODE CUP OVER A LARGE AREA AND THAT THE CAVITY OF THE CATHODE CUP IS DEFORMED IN SUCH A MANNER THAT THE EMISSION BODY IS PERMANENTLY CLAMPED INTO THE CAVITY OF THE CATHODE OF CUP.

Nov. 30, 1971 w. HELD 3,623,198

PROCESS FOR MAKING A THERMIONIC CATHODE' Filed May 5, 1969 2 Sheets-Sheet 1 mvsuron Walter Held Nov. 30, 1971 w. HELD 3,623,198

PROCESS FOR MAKING A THERMIONIC CATHODE Filed May 5, 1969 2 Sheets-Sheet 2 uvvnvron Walter Held %MM l ATTORNEYS United States Patent U.S. Cl. 2925.18 4 Claims ABSTRACT OF THE DISCLOSURE A process for making a thermionic cathode for use in the emission system of a cathode ray tube, the cathode being composed of a sintered pressed body which is to be the electron emitting element and which is formed by pressing powdered material of which the body is to be made into a cavity of an appropriately shaped cathode cup in such a manner that the emission body is formed to communicate with the cathode cup over a large area and that the cavity of the cathode cup is deformed in such a manner that the emission body is permanently clamped into the cavity of the cathode cup.

BACKGROUND OF THE INVENTION T he present invention relates to a process for making a thermionic cathode composed of a body of electron emitting material constructed from a molded piece and a cathode cup, the process being carried out with the aid of a pressing tool having an opening for receiving the cathode cup.

Thermionic cathodes are widely employed in cathode ray tubes and, according to one known manner of construction, are provided with a porous sintered electron emission body which is fastened to a cathode cup containing a heater element. Such cathodes are generally fabricated by completely forming the sintered body in a separate process and then fastening this body to the end of the cathode cup by soldering, pressing, welding or the like. This manner of construction is relatively complicated. Moreover, the manufacture of such cathodes generally requires the use of some type of binder means whose complete subsequent removal is not always assured.

SUMMARY OF THE INVENTION It is a primary object of the present invention to overcome these drawbacks and difficulties.

Another object of the invention is to simplify, reduce the cost of, and reduce the time required for, the production of thermionic cathodes of the above-described type.

A further object of the invention is to provide an improved thermionic cathode.

Yet another object of the present invention is to provide a method for making a novel thermionic cathode which is particularly suited for automated production techniques.

Still another object of the invention is to mass produce thermionic cathodes having a high degree of mechanical and electrical uniformity.

A still further object of the invention is to provide thermionic cathodes having a high emission efficiency.

These and other objects according to the invention are achieved by a process for fabricating a thermionic cathode composed of a pressed electron emission body and a cathode cup one end of which defines a receptacle having a closed bottom. The process includes the step of placing the cathode cup in a mating bore formed in a pressing die, introducing particulate material of which the emission body is to be made into the bore, at the end thereof toward which the stop of the receptacle is directed, and forcing the particulate material against the cathode cup. The forcing is efiected by a pressing stamp in such a manner as to compress the material into a tablet which constitutes the emission body and which fits tightly into the receptacle and to deform the lateral wall of the receptacle in a direction to cause that wall to mechanically clamp the resulting tablet.

Significant advantages of the method according to the present invention are that it requires but a few tools and can be carried out in a short time since the formation of the emission body and its attachment to the cathode cup are accomplished in a single processing stage.

A further advantage of the method according to the present invention is that it eliminates the use of binder means or the like, since the initial material from which the emission body is to be formed may be present in dry powder form. It can therefore consist of the desired material in pure form, i.e. not containing any additives.

Moreover, with the method of the present invention the emission body need not have auxiliary properties, e.g. solderability and weldability, so that one has a completely free hand in this respect in the selection of the emission material.

Further advantages of the method according to the present invention are that it provides an extremely firm anchoring of the emission body and a very good heat transfer connection between the emission body and the cathode cup, this being of decisive significance, in the case of mass production, for achieving uniformity in the emission characteristics of the cathodes, as well as for giving the cathodes a high emission efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, partly in cross section, showing an initial stage in the process according to the invention.

FIG. 2 is a view similar to that of FIG. 1 showing a terminal stage of each process.

FIG. 3 is a longitudinal cross-sectional view of the resulting cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an initial stage in the process of the invention in which a deep drawn cathode cup 2 having one end closed is inserted into the bore of a pressing, or sintering, die 3. The cup 2 has been preliminarily formed so that a portion of its cylindrical surface adjacent its closed end is doubled over to form a receiving chamber. The lateral wall of this chamber is initially cylindrical and initially lies flush against the adjacent cylindrical wall portion of the remainder of the cathode cup. The ways in which a cathode cup can be formed to have the configuration shown in FIG. 1 are. well known to those skilled in the art.

The cathode cup 2 is inserted into the bore of the pressing die 3 and is held and guided therein by a holder 6 in the form of a punch, or stamp onto which the cathode cup 2 is placed and which is adapted as closely as possible to the dimensions of the heater enclosure defined by the interior of the cathode cup 2. In particular, the holder 6 is arranged so that its upper end bears firmly against the closed end of cup 2. The bore in the pressing die 3 is also adapted as closely as possible to the external lateral dimensions of. the cathode cup 2. Preferably, this bore is cylindrical and the cathode cup also has a circular cylindrical cross section.

The cathode cup 2 is pushed into the opening of the pressing die 3 to a definite predetermined location, with respect to the die, by means of the holder 6, the location being such that the space 4 between the closed bottom of cup 2 and the upper edge of die 3 has an accurately predetermined length, or height. The establishment of this relationship is highly desirable because the space 4 above the cathode cup 2 can then be filled to the top with the powdered material 1 from which the emission body is to be formed, so that the height of the space 4 can be used to determine the amount of material 1 to be used in a simple and accurate manner.

The cathode cup is preferably made of molybdenum. The material 1 is of a substance which is best suited for the emission body of the particular thermionic cathode to be produced. Powdered tungsten was used with advantage, for example, as the material in one embodiment of the process according to the present invention.

This substance can also consist of other metal or metal oxide powders which are usual for making dispenser cathodes, for example powdered molybdenum, nickel, tantal, iridium or rhodium or mixtures thereof.

After the material 1 has been introduced, a pressing stamp 5 is inserted, as shown in FIG. 2, into the bore of the pressing die 3 and is urged in the direction of the arrow 8 to compress the material 1 into tablet form so as to create the emission body 7 which has the form of a porous pressed body. The counterpressure is provided by the holder 6. The pressing stamp 5 is dimensioned to conform closely to the opening in the pressing die 3 so that it may be easily moved therein but has a substantial sealing effect.

In a preferred embodiment the cathode cup had a diameter of about 1.5 mm. and consisted of molybdenum having a wall thickness of about 0.1 mm. Tungsten powder having a particle size of about 3-10 m. was pressed into the receptacle with a pressure of about 8-10 kg. per square mm. for a time of about 1 second.

Upon completion of this pressing process it is advisable to loosen the holder 6 and to push the cathode cup 2, together with the emission body 7 attached thereto, out of the bottom of the bore in the pressing die 3. This is preferably followed by subjecting the cathode to a heating process which produces a further hardening of the emission body 7 by a sintering action.

In case of a molybdenum cathode with a tungsten tablet of about 1.5 mm. diameter a sinter temperature was chosen of about 1800-2200 C. for about 520 minutes in a dry hydrogenium atmosphere.

The process according to the invention is particularly advantageous when use is made to a cathode cup 2 having the particular form shown in FIGS. 1 and 2, wherein the I frontal wall of the cathode cup 2 is folded in to form a small cup-shaped area which serves to receive the emission body 7. This folded-in shape of the cathode body has the advantage of assuring that the emission body 7 is hermetically sealed off from the heater chamber of the cathode cup 2. A further advantage of this form of construction is that such a cathode cup may be produced in this manner simply by pressing or drawing and this considerably facilitates mass production.

A very significant advantage presented by the abovedescribed cathode sheath is that during the pressing process shown in FIG. 2, for forming the emission body 7, the cathode cup is deformed in the manner shown so that the lateral wall of the small cup-shaped area at the front end of the cathode cup is deformed under the influence of the force applied by the pressing stamp 8. More specifically, this wall is caused to buckle inwardly to form a loop so that the inner diameter of the surface gripping the emission body 7 is made smaller at its edge than at its bottom. The curvature created in the double wall by the process step shown in FIG. 2 thus produces a firm mechanical anchoring of the emission body 7 so that it could not possibly fall out of the cathode cup.

It has been shown by photomicrographs that the deformation of the cathode cup and the production of the emission body according to the process of the present invention result in an extremely uniform and firm anchoring between the cathode cup and the emission body over practically the entire area of contact between them. The advantage which results from this is that an extremely good heat transfer is assured from the cathode cup to the emission body. A further sintering, for hardening the emission body 7, moreover results in a further strengthening of the bond between the emission body and the cathode cup.

FIG. 3 shows the completed cathode after it has been taken out of the pressing die. It can be seen how the pressed body 7 has become joined with the cathode cup 2 and how the cathode cup 2 has been deformed, at its emission side, to form the curved portion 9 which defines an annular cavity 11. The area for the heater which is to heat the cathode to produce electron emission, is indicated at 10 and it can be seen that this heater chamber 10 is hermetically sealed off from the emission body.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations.

I claim:

1. A process for fabricating a thermionic cathode composed of a pressed electron emission body and a cathode cup one end of which defines a receptacle having a closed bottom, comprising the steps of:

(a) placing the cathode cup in a mating bore in a pressing die;

(b) introducing particulate material of which the emission body is to be made into the bore, at the end thereof toward which the top of the receptacle is directed and;

(c) forcing the particulate material against the cathode cup by means of a pressing stamp in such a manner as to compress the material into a tablet which constitutes the emission body and which fits tightly into the receptacle and to deform the lateral wall of the receptacle inwardly to cause that wall to mechanically clamp the resulting tablet.

2. A process as defined in claim 1 comprising the further step of heating the resulting cathode to effect a sintering action on the tablet.

3. A process as defined in claim 1 wherein the cathode cup is initially present in the form of a deep cup having a closed end which is pushed in to form the receptacle.

4. A process as defined in claim 3 wherein the cathode cup is composed of the closed bottom and a lateral wall, the lateral wall being composed of a first part adjacent the closed bottom and a second part contiguous with the first part, the first part being doubled back to form the lateral wall of the receptacle and to initially lie flush against at least a portion of the second part, which portion is adjacent to, and coextensive with, the first part, and said step of forcing the material against the cup to deform the lateral wall of the receptacle causes an annular space to be created between the receptacle lateral wall and said portion of the second part of the lateral wall.

References Cited JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner US. Cl. XR. 29- 45.17, 420 

